用jpa和hibernate在spring中惰性加载实体

Question

<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xmlns:mvc="http://www.springframework.org/schema/mvc"
    xmlns:context="http://www.springframework.org/schema/context"
    xmlns:tx="http://www.springframework.org/schema/tx"
    xsi:schemaLocation="http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans-3.0.xsd
        http://www.springframework.org/schema/mvc http://www.springframework.org/schema/mvc/spring-mvc-3.0.xsd  
        http://www.springframework.org/schema/context http://www.springframework.org/schema/context/spring-context-3.0.xsd
        http://www.springframework.org/schema/tx http://www.springframework.org/schema/tx/spring-tx.xsd
        http://www.springframework.org/schema/data/jpa
        http://www.springframework.org/schema/data/jpa/spring-jpa.xsd" default-autowire="byName">

    <!-- Scans within the base package of the application for @Components to configure as beans -->
    <bean id="placeholderConfig"
        class="org.springframework.beans.factory.config.PropertyPlaceholderConfigurer">
        <property name="location" value="classpath:db.properties" />
    </bean>
    <tx:annotation-driven transaction-manager="transactionManager"/>
    <!-- <bean id="transactionManager" class="org.springframework.orm.jpa.JpaTransactionManager">
    <property name="entityManagerFactory" ref="jpaVendorAdapter" />
    </bean> -->
    <bean id="entityManagerFactory"
        class="org.springframework.orm.jpa.LocalContainerEntityManagerFactoryBean">
        <property name="jpaVendorAdapter">
            <bean class="org.springframework.orm.jpa.vendor.HibernateJpaVendorAdapter">
                <property name="showSql" value="true" />
                <property name="generateDdl" value="true" />
                <property name="databasePlatform" value="${db.dialect}" />
            </bean>
        </property>     
    </bean>

    <bean id="dataSource" class="org.apache.commons.dbcp.BasicDataSource"
        destroy-method="close">
        <property name="driverClassName" value="${db.driver}" />
        <property name="url" value="${db.url}" />
        <property name="username" value="${db.username}" />
        <property name="password" value="${db.password}" />
    </bean>

    <bean id="transactionManager" class="org.springframework.orm.jpa.JpaTransactionManager" />
</beans>

@Transactional(readOnly = true, propagation=Propagation.REQUIRED)

@Controller
public class HomeController{

    @Autowired
    private UserDao userDao;

    @RequestMapping(value = "/", method = RequestMethod.GET)
    public ResponseEntity<String> home(){
        ...
        User user = userDao.findUser(id);
        Set<Order> orders = user.getOrders();
        ...
    String myResult = ...;
    return jsonService.generateResponse(myResult);
    }

}

@Repository
public class UserDao{

    @PersistenceContext
    private EntityManager entityManager;

    public User findUser(Integer id){
        return entityManager.find(User.class, id);
    }
}

这组订单应该是惰性加载的，但我得到以下异常：org.springframework.web.util.NestedServletException：请求处理失败；嵌套异常是org.hibernate.lazyInitializationException：未能懒洋洋地初始化Role:...，没有会话或会话被关闭

根本原因：org.hibernate.lazyInitializationException：未能懒洋洋地初始化Role:...的集合，没有会话或会话被关闭

我尝试在控制器wirt@Transactional中注释该方法，并在db.xml中的注释驱动属性中设置mode=“AspectJ”，但没有任何效果。有什么方法可以延迟加载用户的订单吗？

对您的任何帮助，在期待中感谢您！

Answer 1

您可以使用特殊筛选器在web视图中获取会话。添加到web.xml

<filter>
    <filter-name>jpaFilter</filter-name>
    <filter-class>org.springframework.orm.jpa.support.OpenEntityManagerInViewFilter</filter-class>
</filter>
<filter-mapping>
    <filter-name>jpaFilter</filter-name>
    <url-pattern>/*</url-pattern>
</filter-mapping>

过滤器的工作方式如下：

• 过滤器拦截servlet请求
• 筛选器打开EntityManager并将其绑定到当前线程
• Web控制器称为
• Web控制器调用服务
• 事务拦截器开始一个新事务，检索线程绑定的EntityManager并将其绑定到事务
调用
• 服务，对EntityManager执行一些操作，然后返回
• 事务拦截器刷新EntityManager，然后提交事务
• Web控制器准备视图，然后返回
• 视图已生成
• 筛选器关闭EntityManager并将其与当前线程解除绑定